/**
   * Returns the string value of the first of the properties in the uriDescriptionList for the given
   * resource (as an URI). In case the resource does not have any of the properties mentioned, its
   * URI is returned. The value is obtained by querying the endpoint and the endpoint is queried
   * repeatedly until it gives a response (value or the lack of it)
   *
   * <p>It is highly recommended that the list contains properties like labels or titles, with test
   * values.
   *
   * @param uri - the URI for which a label is required
   * @return a String value, either a label for the parameter or its value if no label is obtained
   *     from the endpoint
   */
  private String getLabelForUri(String uri) {
    String result;

    if (uriLabelCache.containsKey(uri)) {
      return uriLabelCache.get(uri);
    }

    for (String prop : uriDescriptionList) {
      String innerQuery = "SELECT ?r WHERE {<" + uri + "> <" + prop + "> ?r } LIMIT 1";

      try {
        Query query = QueryFactory.create(innerQuery);
        QueryExecution qExec = QueryExecutionFactory.sparqlService(rdfEndpoint, query);
        boolean keepTrying = true;
        while (keepTrying) {
          keepTrying = false;
          try {
            ResultSet results = qExec.execSelect();

            if (results.hasNext()) {
              QuerySolution sol = results.nextSolution();
              result = EEASettings.parseForJson(sol.getLiteral("r").getLexicalForm());
              if (!result.isEmpty()) {
                uriLabelCache.put(uri, result);
                return result;
              }
            }
          } catch (Exception e) {
            keepTrying = true;
            logger.warn("Could not get label for uri {}. Retrying.", uri);
          } finally {
            qExec.close();
          }
        }
      } catch (QueryParseException qpe) {
        logger.error("Exception for query {}. The label cannot be obtained", innerQuery);
      }
    }
    return uri;
  }
  /**
   * Get a set of unique queryObjName returned from a select query
   *
   * <p>Used to retrieve sets of modified objects used in sync
   *
   * @param rdfQuery query to execute
   * @param queryObjName name of the object returned
   * @return set of values for queryObjectName in the rdfQuery result
   */
  HashSet<String> executeSyncQuery(String rdfQuery, String queryObjName) {
    HashSet<String> rdfUrls = new HashSet<String>();

    Query query;
    try {
      query = QueryFactory.create(rdfQuery);
    } catch (QueryParseException qpe) {
      logger.warn(
          "Could not parse [{}]. Please provide a relevant query. {}",
          rdfQuery,
          qpe.getLocalizedMessage());
      return null;
    }

    QueryExecution qExec = QueryExecutionFactory.sparqlService(rdfEndpoint, query);
    try {
      ResultSet results = qExec.execSelect();

      while (results.hasNext()) {
        QuerySolution sol = results.nextSolution();
        try {
          String value = sol.getResource(queryObjName).toString();
          rdfUrls.add(value);
        } catch (NoSuchElementException e) {
          logger.error("Encountered a NoSuchElementException: " + e.getLocalizedMessage());
          return null;
        }
      }
    } catch (Exception e) {
      logger.error(
          "Encountered a [{}] while querying the endpoint for sync", e.getLocalizedMessage());
      return null;
    } finally {
      qExec.close();
    }

    return rdfUrls;
  }